This invention relates to the continuous slab casting of steel. In the continuous slab casting of steel, molten steel from a steelmaking ladle is poured indirectly through a subentry nozzle into an oscillating casting mold, and the steel is continuously cast in a semi-finished strand to make a slab, bloom, or billet. The semi-finished shape of the strand is determined by the continuous casting mold with a molten inner core and a solidified outer surface as the strand moves downwardly through the mold. The strand is subjected to secondary cooling upon exiting from the mold until the entire strand is solidified. The strand is then cut into slabs, blooms, or billets.
In the continuous caster, the molten steel flows from the tundish into the mold through a submerged entry nozzle (SEN). The SEN discharges the molten metal into the mold to a selected depth below the surface (the “meniscus”) of the melt in the mold. The flow of the molten melt from the tundish is gravitationally fed by the pressure difference between the liquid levels of the tundish and that of the melt in the mold. The melt flow from the tundish may be controlled by a stopper rod which at least partially blocks the exit port to the SEN, or a slide gate that moves across the outlet port of the tundish to the SEN. As the molten metal enters the mold, the steel solidifies at the water cooled mold walls to form a shell, which is continuously withdrawn at the casting speed to produce the steel strand by oscillation of the mold walls.
One of the prime difficulties in such continuous casting of steel is having a uniform and consistent mold heat transfer rate, which affects the final casted steel. The origin of the majority of surface defects in the cast strip is at, or within, a very short distance of the meniscus in the mold. Whether the defects propagate into cracks depends on the heat transfer in the remainder of the mold and events and conditions at and below the mold exit. As such, regulating the heat transfer rate is essential.
The heat transfer rate is can be affected by the amount of dissolved gases, particularly hydrogen, in the molten metal. As such, fluctuation in hydrogen levels in the molten metal may cause defects in the steel product and even breakouts as the steel is casted, which in turn, would increase maintenance costs and decrease productivity.
Excessive hydrogen concentrations may decrease the heat transfer rate through the liquid metal causing various defects and risk possible breakouts in the mold. Due to its high mobility, hydrogen can easily diffuse through the lattice of the steel microstructure. Hydrogen may be picked up by the molten metal through steelmaking additions or processes, several of these could be hydrated lime, wet alloys or excessive furnace slag carry over. Hydrogen may also be picked up by the molten metal from the atmosphere. As shown in FIG. 11 of the paper titled “Hydrogen and Nitrogen in Steel Making at U.S. Steel” published in the November 2009 issue of Iron & Steel Technology, higher humidity conditions characteristic of steelmaking facilities located in northern U.S. resulted in higher average hydrogen levels.
Conversely, abnormally low hydrogen concentrations may increase the heat transfer rate of the liquid metal causing various casting defects, such as surface cracks in the finished product. Decrease in hydrogen levels is aggravated by cold and dry weather conditions. It is known that the extent of hydrogen pickup strongly depends on the partial pressure of water vapor (i.e. humidity) in the atmosphere. Since the amount of moisture in the air depends on the temperature and the relative humidity, cold and dry winter days provide conditions for unusually low hydrogen levels when compared to summer days.
Currently, there are known methods for regulating the heat transfer rate by altering the casting conditions in the mold. One of these methods is the variation of the physical composition of the mold powder at the continuous caster. The physical components of the mold powder affects the heat transfer rate. As the mold powder melts and solidifies in the mold, the mold powder interacts with the hydrogen in the molten steel and the glass state of the solidified mold powder is altered, affecting the heat transfer rate of said powder. However, developing mold powders dependent on hydrogen levels requires having an extensive inventory of mold powders of different compositions. Additionally, it requires supervision by a trained operator to select the correct mold powder from the various mold powder compositions to control the hydrogen levels in the mold with different operational conditions.
Previous methods control hydrogen levels by modifying the refining process or by using a downstream degasser. Degassing the steel has proven effective in reducing the hydrogen levels and altering the method and timing of the alloy additions have also proved effective in reducing the hydrogen levels in the produced steel.
Accordingly, there remains a need for a method for increasing the hydrogen levels in steel composition for consistent heat transfer in continuous casting that is both effective and economical.
Presently disclosed is a method of controlling the amount of hydrogen in steel for consistent heat transfer in continuous casting by adding a hydrocarbon to the molten metal. Disclosed is a method of continuous casting comprising the steps of:                a. forming a heat of molten steel in a ladle metallurgy furnace adapted for use in continuous casting;        b. adding a hydrocarbon to the molten metal in the ladle metallurgy furnace in an amount sufficient to increase hydrogen levels in the molten steel for casting; and        c. delivering the molten steel with a desired level of hydrogen to a caster to continuously cast a steel product.        
The hydrocarbon may be delivered to the molten steel in the ladle metallurgy furnace in an amount sufficient to provide between 5 and 9 ppm of hydrogen in the molten steel delivered to the caster for continuous casting into a steel product. Alternatively, the hydrocarbon may be delivered to the molten steel in the ladle metallurgy furnace in an amount sufficient to provide between 6 and 8 ppm of hydrogen in the molten steel delivered to the caster for continuous casting into a steel product.
The hydrocarbon may be methane. The hydrocarbon may be delivered to the molten metal in the ladle metallurgy furnace by bottom stirring. In some embodiments, the hydrocarbon may be stirred at a rate of 15 SCFM. In other embodiments, the hydrocarbon may be stirred at a rate of 20 SCFM.
Also disclosed is a method of continuously casting comprising the steps of:                a. forming a heat of molten metal in a ladle metallurgy furnace adapted for use in continuous casting;        b. adding a hydrocarbon to the molten metal in the ladle metallurgy furnace in an amount sufficient to increase hydrogen levels in the molten metal;        c. assembling a casting mold for continuous casting;        d. introducing the molten metal into the casting mold and forming a cast strand;        e. delivering the cast strand to a support roller assembly for cooling; and        f. forming a slab of continuous cast steel product.        
The hydrocarbon may be delivered to the molten steel in the ladle metallurgy furnace in an amount sufficient to provide between 5 and 9 ppm of hydrogen in the molten steel delivered to the caster for continuous casting into a steel product. Alternatively, the hydrocarbon may be delivered to the molten metal in the ladle metallurgy furnace in an amount sufficient to provide between 6 and 8 ppm of hydrogen in the molten steel delivered to the caster for continuous casting into a steel product.
The hydrocarbon may be methane. The hydrocarbon may be delivered to the molten metal in the ladle metallurgy furnace by bottom stirring. In some embodiments, the hydrocarbon may be stirred at a rate of 15 SCFM. In other embodiments, the hydrocarbon may be stirred at a rate of 20 SCFM.